The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Wind turbine technology is a rapidly developing field of technology. The installation and operation of a wind park and the connection of such a wind park to the public grid however provides some technological challenges. Wind turbines may be installed in areas wherein the grid is not very stable, so that the control system of the wind farm and/or the control system in each wind turbine of the wind farm should be able to take measures when so-called grid faults or faults occur during operation. Here, a fault may be defined as a utility disturbance that results in the voltage on the utility transmission system (e.g. the public grid) to drop by a significant amount for a short duration (typically less than 500 ms). Faults may be caused by ground connections and short circuits and often appear during lighting and storms. A significant voltage drop may also appear due to a large change in the load in the utility transmission system.
Conventionally, a turbine may be protected against such grid faults using a fault protection circuit. When a fault is detected, the fault detection circuit may for example dump the excess power generated by the turbine during the fault into a number of resistor banks. Once the resistor banks are overloaded, the power and associated torque are reduced to zero in a couple of milliseconds. A similar situation may occur when an emergency stop in a wind turbine is triggered. Such emergency stop may e.g. be triggered if a particular turbine element is defect. Also in that case, the torque of the wind turbine is reduced to zero within milliseconds.
A sudden decrease of the wind turbine torque exposes the turbine and the tower to severe mechanical stress. Especially in areas wherein grid faults occur frequently, the effects of frequent torque shut downs may eventually have detrimental mechanical effects to the wind turbine structure.
Hence, there is a need in the prior art for improved torque control during grid faults and emergency stops. In particular, there is a need for torque control, which mitigates the problems related to the exposure of the wind turbine to heavy mechanical loads.